1. Field of the Invention
The present invention relates to oxadiazole derivatives. Furthermore, the invention relates to light-emitting elements, light-emitting devices, and electronic devices in which oxadiazole derivatives are used.
2. Description of the Related Art
There is a greater variety in types of organic compounds compared to inorganic compounds, and materials that have a variety of different functions can be designed and synthesized. Because of these types of advantages, attention has been focused on electronics devices in which organic compounds are used in recent years. For example, solar cells, light-emitting elements, transistors, and the like in which organic compounds are used as functional materials are some typical examples of these electronics devices.
These electronics devices are devices that use the electrical properties and optical properties of organic compounds; of these devices, in particular, research and development of light-emitting elements in which organic compounds are used as luminescent materials has been showing an impressive amount of progress.
The structure of these light-emitting elements is a simple structure in which only a light-emitting layer that contains an organic compound, which is a luminescent material, is provided between electrodes, and these light-emitting elements have been attracting attention as elements of next-generation flat panel display panels for having the characteristics of being thin and light-weight, having high-speed response, using low direct current low voltage driving, and the like. Furthermore, displays in which these light-emitting elements are used also have the characteristics of having superior contrast and image quality and having a wide viewing angle.
The light-emitting mechanism of the light-emitting elements in which organic compounds are used as the luminescent material is carrier injection. That is, by application of a voltage to a light-emitting layer that is interposed between electrodes, holes and electrons injected from the electrodes recombine to place the luminescent material into an excited state, and the luminescent material emits light when the luminescent material returns to the ground state from the excited state. For types of excited states, there is the singlet excited state (S*) and the triplet excited state (T*). In addition, it is thought that the statistical generation ratio of the two states in a light-emitting element is S*:T*=1:3.
For compounds that emit light when the material returns to the ground state from the singlet excited state (hereinafter, these compounds will be referred to as fluorescent compounds) at room temperature, no emission of light from the triplet excited state (emission of light by phosphorescence) is observed, and only emission of light from the singlet excited state (emission of light by fluorescence) is observed. Consequently, the logical limitation on the internal quantum efficiency (the ratio of the number of photons generated with respect to the number of carriers injected) in a light-emitting element in which a fluorescent compound is used is regarded as being 25%, based on S*:T*=1:3.
On the other hand, if compounds that emit light when the material returns to the ground state from the triplet excited state (hereinafter, these compounds will be referred to as phosphorescent compounds) are used, the internal quantum efficiency could theoretically be from 75% to 100%. That is to say, a luminous efficacy of phosphorescent compounds could be three to four times as great as that of fluorescent compounds. For these kinds of reasons, for the realization of high-efficiency light-emitting elements, development of light-emitting elements in which phosphorescent compounds are used is being actively carried out in recent years (referring to Non-Patent Document 1 for examples).
When the light-emitting layer of a light-emitting element is formed using one of the aforementioned phosphorescent compounds, for suppression of the concentration quenching of the phosphorescent compound and the quenching due to triplet-triplet annihilation, there are many cases in which the light-emitting layer is formed so that the phosphorescent compound is dispersed throughout a matrix formed of another material. In this case, the material used to form the matrix is called a host material, and the material dispersed throughout the matrix like the phosphorescent material is called a guest material.
When a phosphorescent compound is used for the guest material, a property required of the host material is that the host material have a greater triplet excitation energy (the difference in energy between the ground state and the triplet excited state) than that of the phosphorescent compound. CBP, which is used as the host material in Non-Patent Document 1, is known to have a greater triplet excitation energy than a phosphorescent compound that exhibits emission of light of a green to red color and is widely used as a host material with the phosphorescent compound.
[Non-Patent Document 1] M. A. Baldo et al., Applied Physics Letters 75, no. 1 (1999): 4-6.